I have a strange problem with a calculation. It seems to be working
for most "rows" in my table, but one set of rows fails, and I can't
figure out why. Can someone help me troubleshoot? Here's the set up.

I have two tables. The first, called COURSE, has two fields: Course
and Credit. The latter field lists the number of credits a student
earns for taking the course in Course. The Course field is related to
the Course field in another table. This second table, ALL, has a field
Students that stores the number of students. It also has fields for
Semester and year. What I want to calculate is the number of credits
generated by each course each semester, so I use a calculation field in
the ALL table that looks like "Students*COURSE::Credit". For every
course but one, this gives me the correct number, but for one course, I
get nothing. No number. When I fiddle with the course information in
the COURSE table (change the number of credits, reenter it into the
table, etc.) nothing happens. When I delete the row in COURSE that
defines that course, nothing changes, either. What could be going
wrong?

Thanks for whatever advice people can share. I know it's probably a
dumb question, but none of my references are any help.

If I understand correctly then your structure should be setup as:

COURSES
ID Course
Course Name
Number Of Credits
Selected Year_g (global field)
Selected Semester_g (global field)

STUDENTS
ID Student
Name

COURSE_STUDENT (cross table contains 1 record / student / course)
ID Student
ID Course
Year
Semester


To get the count of credits for a specific semester you would create a
new table occurrence of "Course_Student", call it "CS" and link it to
"Courses" so that
Courses::ID Course = CS::ID Course
Courses::Selected Year_g = CS::Year
Courses::Selected Semester_g = CS::Semester

Now you can create a calc in courses that gives you the correct count
whenever you select the year and semester. The calc =
Count(CS::ID Course) * Number of Credits

Note the count function could use any field in the CS table reference
that has data in it because it counts the record as 1 if it has data
and as 0 if it doesn't.